The challenge and importance of predicting sea-level rise

By Prof Tim Naish, Director of the Antarctic Research Centre, Victoria University Wellington

By the end of the century it is predicted that rising sea-level will directly impact more than 200 million people around the world. The damage to property and infrastructure, groundwater, and the prospect of widespread displacement of people will indirectly affect every one of us on this planet.

The 2 million Syrian refugees have given us a small taste of the disruption that will be caused by climate change. So how fast will sea-levels rise? What are the uncertainties facing the science of sea-level rise prediction? What are the challenges facing New Zealand?

After assessing the evidence, the Intergovernmental Panel on Climate Change (IPCC) noted, but did not include in their global sea-level predictions, the potentially large contribution from rapid retreat of unstable parts of the Antarctic ice sheet. They argued at the time of writing, that the scientific evidence was not clear enough for quantifying the likelihood of a rapid and potentially non-linear response by Antarctica, but cautioned that “based on current understanding, collapse of marine-based sectors of the Antarctic ice sheets, if initiated, could cause global mean sea level to raise tens of centimetres above the “likely” range [of up to 98cm] during the 21st century”. This statement highlights one of the biggest uncertainties in climate change science, and it is of immediate concern to decision-makers and the public.

Vulnerable low-lying island and coastal nations were so concerned about sea-level rise, they would not sign the Paris Climate Agreement unless the IPCC took a closer look at limiting global warming to 1.5°C. I attended the IPCC’s meeting held in Geneva last September to scope the content of the 1.5°C report, and returned with the sad realization that at our current rate of carbon dioxide emissions, we will pass 1.5°C within the next 5 years. The report will not be completed for 3 years!

The future fate of the Antarctic ice sheet is the gorilla in the room.

How much?

NASA scientist, Dr Eric Rignot, who will visit Wellington to give a lecture on the 14th of February, suggests that it is now too late for a large part of West Antarctic Ice Sheet, which could raise sea-level by 3m. Dr James Hansen, former Director of NASA’s Goddard Space Centre, and a highly respected atmospheric scientist, has argued based on the current acceleration in the rate of the polar ice sheets melting, global sea-levels could rise more than 2m above by the end of the century.

However, IPCC do not base their predictions on extrapolations of relatively short-trends. Rather, they use computer simulations of the physical processes that occur in an ice sheet in response to climate warming. Since the IPCC report in 2013, scientists have made significant progress using ice sheet models to better capture the key processes that lead to rapid loss of the Antarctic ice. Results from a study published last year in Nature, led by Dr Nick Golledge at Victoria University of Wellington, show higher sea-level contributions from Antarctic ice melting for the range of IPCC future climate scenarios above 2°C. The bottom line is that the current sea-level projections may significantly underestimate the Antarctic contribution to future sea level by 2100 by as much as 80cm for the business as usual scenario without emissions reductions.

However, there is also a good news story in this new research. The results of the Golledge study, and a second study published in Nature last year by our American collaborators, Professors Rob DeConto and David Pollard, show that stabilization of earth’s temperature at 2°C above pre-industrial levels, the target signed up to by 197 countries in the Paris agreement, saves the Antarctica ice sheet from significant melting and dramatically improves the prospects for island nations. In other words, there appears to be a threshold in the Antarctic ice sheet around 2°C of global warming.

Victoria University Wellington

Sea-level rise in New Zealand

So what does sea-level rise mean for New Zealand, and why should we care? In a recent report, the NZ Parliamentary Commissioner for the Environment, made the following statement. “It is certain that the sea is rising and will continue to do so for centuries to come. But much is uncertain – how rapidly it will rise, how different coastal areas will be affected, and how we should prepare.”

The report noted that local factors matter, and in New Zealand vertical land movements caused by tectonic processes must be taken into account. While our biggest city Auckland is more or less tectonically stable, the lower North Island is subsiding by up to 3mm per year, which means Wellington could experience 30cm of additional sea-level rise above the global average predicted over the next 100 years – a worst case scenario might sea sea-level nearer 2m if new Antarctic melt estimates are also included. Subsidence of south Dunedin combined with sea-level rise, will result in the 100-year coastal flooding event occurring several times a year by mid-century. Changes in sediment deposition and ongoing subsidence associated with the Christchurch earthquakes has significantly increased the risk of coastal flooding for large parts of Christchurch.

Spatially variable land subsidence provides an additional challenge for New Zealand local authorities charged with building resilience and adapting to sea-level rise in their district plans. Rising seas must be considered for regulating green fields developments and future proofing infrastructure. In the case of some councils (Kapiti and Christchurch) opposition to proposed zoning has seen the underpinning science questioned in court.

The Ministry for the Environment has just updated its “Coast Hazard Guidance” for local authorities based on the latest scientific evidence, which will include high emissions, sea-level rise scenario that takes into account some of the latest Antarctic estimates. While, the role of local ground subsidence on sea-level changes remains poorly understood, and more scientific research is urgently required, local authorities are advised by the MfE guidance, to account for land subsidence in their district and city plans.

Climatic futures

For now, the range of possible climatic futures remains large, due to a lack of certainty over the collective international ability to deliver emission reductions. Planning for future sea-level rise in New Zealand will require a flexible and precautionary approach. Sea-level rise over the next 50 years is relatively well-known and is in the range of 30-40cm under all scenarios. This is already built in from the climate change we have had, so far. Although the scientific understanding will continue to improve, global mitigation measures taken over the next 2 decades will largely control how much sea-level rise we are committed to by the end of the century and beyond, and whether we commit our planet to the melt down of the polar ice sheets.

The Sciblogs Horizon Scan

This post is part of the Sciblogs Horizon Scan summer series, featuring posts from New Zealand researchers exploring what the future holds across a range of fields.

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